Electron oribitals

Question 1

How did scientists determine that electrons occupy specific energy levels as they travel around the atomic nucleus

Answer 1

The more they knew about the position of a quantum particle the less they knew about the motion in, (and vise versa), so they used that for figuring out the placement of the elections

Question 2

Why does the shape of the periodic table corresponds to the electron energy levels

Answer 2

Because it shows which electrons have their valence shells filled the most